A multi-carrier wireless transmission system is implemented based on an Orthogonal Frequency Division Multiplexing (OFDM) technology. The OFDM, served as a high-speed transmission technology for combating multi-path fading, partitions channels into many orthogonal sub-channels in frequency domain; the carriers of various sub-channels keep orthogonal; a high-speed data flow is transmitted at a low bit rate after being converted to such orthogonal and parallel multiple sub-carriers by serial-to-parallel conversion.
Since a wireless channel is always a fading channel, it is necessary for inserting pilots in the sent data signals according to a preset pilot format in a multi-carrier wireless transmission system, so that the receiving party can perform the real-time estimation and tracking for the channel according to the received pilot signal. A pilot is a signal known by a receiving party. If no explicitly express, a pilot may be call as a pilot signal in this present invention.
FIG. 1 is a schematic diagram illustrating a wireless time-frequency transmission block and pilot format of multi-carrier wireless transmission system in the prior art. Referring to FIG. 1, in the present multi-carrier wireless transmission system such as mobile WiMAX evolution system, a data transmission unit sent by each sector/cell is a wireless time-frequency transmission block as shown in FIG. 1; the wireless time-frequency transmission block is composed of six consecutive OFDM symbols in time domain (horizontal axis direction) and eighteen consecutive physical sub-carriers in frequency domain (longitudinal axis direction).
In the prior art, a pilot format is designed for the wireless time-frequency transmission block as shown in FIG. 1. The pilot format is that, three pilot signal transmission areas (each pilot signal transmission area occupies two physical sub-carriers) are set in a wireless time-frequency transmission block of each sector/cell; in each pilot signal transmission area, each antenna of sector/cell sends two pilot signals of the antenna by use of two minimal time-frequency cell-grids (the minimal time-frequency cell-grid is a minimal unit of wireless transmission resource composed of one OFDM symbol and one physical sub-carrier, e.g. an area marked 1 in FIG. 1 is a minimal time-frequency cell-grid and an area marked 5 is also a minimal time-frequency cell-grid). For example, FIG. 1 is a schematic diagram for transmitting signals by use of a wireless time-frequency transmission block. The wireless time-frequency transmission block includes three pilot signal transmission areas; pilot signals of antennas 1-6 of sector/cell 1 are respectively transmitted by minimal time-frequency cell-grids marked with 1-6; in the case that three adjacent cells constructs a cell group, cell 1 may transmit pilot signals of antennas 1-2 respectively by use of minimal time-frequency cell-grids marked with 1-2, cell 2 may transmit pilot signals of antennas 1-2 respectively by use of minimal time-frequency cell-grids marked with 3-4, and cell 3 may transmit pilot signals of antennas 1-2 respectively by use of minimal time-frequency cell-grids marked with 5-6. If there are multiple antennas in a cell, to avoid the interference, minimal time-frequency cell-grids occupied by various antennas are usually non-overlapping, as stated above. In addition, as shown in FIG. 1, pilot signals of antenna 1 may be referred to as pilot flow 1; in a similar way, pilot signals of antenna 2 may be referred to as pilot flow 2.
In fact, in an existing multi-carrier wireless transmission system, when there are multiple cells necessary for transmitting pilot signals and supporting multiple antenna transmission, and such cells are adjacent or overlapping, to reduce the interference to each other, minimal time-frequency cell-grids occupied by various antennas of various cells are usually non-overlapping, i.e. various pilot flows of various cells are non-overlapping. It is because that, the existing pilot signals transmission method is used for transmitting pilot signals of single cell. For a downlink, a receiving party estimates the condition of the downlink come from the local base station of cell by use of pilot signals. Along with the evolution of wireless transmission system, however, more and more services and requirements will be achieved; some services provide the different requests for transmission of pilot signals. In a shared frequency network, for example, in a multicast broadcast service based on multiple cells, since the information sent by multiple base stations are available information, and even the same information, such information should be merged and received to acquire the diversity gain, which is not the same as the case of the above single cell transmission in which the information come from other base stations are regarded as interferences and are eliminated as far as possible. However, when the merged information come from multiple base stations is used as the downlink information, the corresponding downlink condition is relatively complicated; therefore, it is necessary for a better channel estimation, i.e. it provides a higher request for the design of the corresponding pilot signal. At present, there no a better design scheme of pilot signal for solving such a problem.
In addition, in a multi-carrier wireless transmission system, the existing pilot signal transmission method is obviously asymmetric in the time-frequency domain. For example, in a wireless time-frequency transmission block as shown in FIG. 3(b), pilot flow 1 uses minimal time-frequency cell-grid marked as 1, transmits two pilot signals at the first OFDM symbol (the first column from the left), and transmits one pilot signals at the sixth OFDM symbol (the first column from the right) symmetrical along time domain, i.e. it is obviously asymmetric in time domain. Such an asymmetry makes the channel estimation algorithm more complicated, as a result that it is difficult to implement the channel estimation.
Moreover, in a multi-carrier wireless transmission system, the existing pilot signal transmission method doesn't utilize the edge of wireless time-frequency transmission block adequately. For example, in a wireless time-frequency transmission block as shown in FIG. 3(d), pilot flow 1 transmits three pilot signals at the first and the sixth OFDM signals at the edge of time domain by use of minimal time-frequency cell-grid marked with 1; in fact, more pilot signals such as four pilot signals are transmitted, the better the effect is. As shown in FIG. 3(d), since the edge of wireless time-frequency transmission block isn't utilized adequately, the channel estimation of pilot flow 1 at the minimal time-frequency cell-grid of (1,9), i.e. Column 1 Row 9, has to use an extrapolation algorithm, which obviously prejudices the accuracy of channel estimation and increases the implementation complexity.